Relative Heat Gain. The amount of heat gained through glass taking into consideration the effects U-value and shading coefficient. Using the NFRC standard, relative heat gain is calculated as follows: English System: RHG = (Summer U-value x 14°F) + (Shading Coefficient x 200). Metric System: RHG = (Summer U-value x 7.8°C) + (Shading Coefficient x 630). The lower the RHG, the more the glass product restricts heat gain.